Refrigeration systems such as refrigerators, heat pumps, air conditioners and the like use heat exchangers for heating or cooling a region. A heat exchange surface can become covered with frost and then ice as the heat exchanger gathers heat from the surrounding atmosphere. If ice is allowed to accumulate substantially the efficiency of the heat exchanger and the system as a whole is reduced, making it more expensive to operate the refrigeration system. Techniques are known in the prior art for periodically defrosting refrigeration system heat exchangers that have become covered with ice.
One common defrost procedure reverses refrigerant flow through the system. This causes hot refrigeration to flow through the ice-covered heat exchanger and melt accumulated ice from the heat exchanger. Refrigerant flow reversal is usually initiated by a controller coupled to a refrigerant flow reversing valve. The controller outputs a two state control signal where one state causes the refrigeration system to operate in a normal heating or cooling mode. In the second signal state, the reversing valve is actuated to reverse the refrigerant flow and defrost the heat exchanger for a time period sufficient to clear accumulated ice from the heat exchanger. This defrost period is typically substantially shorter than the time it takes for the ice to accumulate. Prior art controllers therefore provide a two state control signal wherein a defrost signal is substantially shorter in duration than the heating/cooling signal.
Another defrosting procedure employs electric heaters which are periodically energized to melt accumulated ice or frost from heat exchangers. These heaters are operated from controllers like the refrigerant flow reversing valves.
U.S. Pat. No. 4,731,729 to Uetrecht et al. discloses a timing control circuit for producing two sequential events having different durations. The disclosed control has particular application in environmental systems. The timing control circuit disclosed in the '729 patent is programmable and contains a single oscillator and a binary counter set to count a predetermined number of oscillator output cycles. At the end of a first count, oscillator tuning components are switched to change the oscillator frequency and the counter is reset. Changing the frequency changes the duration for the next count and the next corresponding event.
U.S. Pat. No. 4,745,629 to Essig et al. discloses a defrost timer circuit having a duty cycle providing alternate ON and OFF intervals corresponding to different timer circuit logic states. The timer is constructed using an integrated circuit having two independent clock sources, each driving a respective counter. While one counter times out, the circuit output is in one state and while the other counter times out, the output is in the other logic state.
The present invention concerns a defrost control apparatus and method for periodically defrosting a refrigeration system heat exchanger. After a predetermined heating or cooling interval the control apparatus initiates a defrost of the heat exchanger for a controlled time interval.